1. Field of the Invention
The present invention relates to silicone rubber which contains one or more types of hollow organic resin filler and have at the interior an open-cell state. The invention also relates to a method of producing such silicone rubber.
2. Prior Art
Heat-curable liquid silicone rubber compositions are used in many different applications since they are effectively moldable and once molded, they provide cured products (silicone rubbers) having excellent qualities, including heat resistance, weather resistance and electrical insulating properties. One distinctive type of silicone rubber having a broad range of potential applications is sponge-like silicone rubber. In addition to possessing the above outstanding performance features of silicone rubbers (heat resistance, weather resistance, electric insulating properties, etc.), silicone rubber sponges can be made lightweight. Moreover, the inclusion of a gas in the molded material provides volumetric shrinkage qualities that enable the silicone rubber sponge to be used as an shock-absorbing, or cushioning, material. The low heat conductivity resulting from the incorporation of a gas also allows silicone rubber sponges to be used as a heat-insulating or heat-storing materials.
Such silicone rubber sponges are produced from silicone rubber compositions to which a blowing agent is added. The blowing agent may be incorporated in a number of ways, such as by adding a heat-decomposable blowing agent or by using hydrogen gas generated as a by-product during curing. Drawbacks of adding a heat-decomposable blowing agent include the toxicity and odor of the decomposition gases. When a platinum catalyst is used as the curing catalyst, an additional problem has been the inhibition of curing by the blowing agent. The use of hydrogen gas generated as a by-product of curing also presents a number of difficulties, such as the explosive nature of hydrogen gas and the special care required when handling the uncured product during storage. Another problem with using hydrogen gas generated during curing as the foaming agent, particularly when the silicone rubber composition is a liquid, is the difficulty of obtaining uniformly controlled cells. One method that provides some improvement in these respects involves incorporating within the silicone rubber composition a hollow powder made of an inorganic material such as glass or ceramic (U.S. Pat. No. 5,580,794). Yet, such inorganic materials have a high specific gravity and thus are not conducive to a sufficient reduction in the weight of the silicone rubber. Moreover, as inorganic materials, they do not adequately lower the heat conductivity and confer on the sponge only poor cushioning properties. According to another prior-art technique, described in JP-A 5-209080 corresponding to U.S. Pat. No. 5,246,973, an organic resin filler which swells under heating is included in the silicone rubber composition. Expansion is induced during curing to give a cell-containing molded product. However, this approach has a number of molding-related drawbacks, including an inability to reliably mold articles of a predetermined size and poor uniformity of curing. Another known method, described in JP-A 9-137063 corresponding to U.S. Pat. No. 5,750,581, provides for the addition of a hollow organic resin filler. However, the resulting molded material contains discrete and independent cells, and thus fails to fully overcome such shortcomings as insufficient cushioning property and a high compression set.